The present invention relates to marine propulsion assemblies. More specifically, the present invention relates to marine drive units having a skeg element that precedes a propeller for steering control, propeller protection and running stability.
Traditionally, outboard and stern marine drives have included a vertical drive shaft surrounded by and aligned within a faired housing that is secured to a vessel transom. The lower end of the drive shaft housing is terminated by a gear case or pinion housing. A propeller mounting arbor is aligned within the gear case and projects from the aft end of the case. The internal end of the arbor carries a pinion gear that meshes with a corresponding drive shaft pinion thereby turning the rotational drive line 90.degree..
Outside of a gear case end wall seal, the projected end of the arbor shaft receives the marine drive propeller by such structural devices as will transmit torque and rotating power to the propeller with accommodation for some degree of shock absorption.
Below the gear case and, traditionally, as an integrally cast extension therefrom, is a radially projecting skeg element. Classically, a skeg is an extended vessel keel that is constructed and positioned to protect the lower rotational arc of a propeller or screw from engaging the bottom of the floatation water body or any submerged obstacles. In an outboard or stern drive, the skeg performs a similar propeller protection function but also functions as a steering rudder. In higher speed ranges, the skeg becomes increasingly important to lateral stability of the vessel and for propeller counter-torque trim.
When a propeller driven light utility or racing vessel achieves speeds in excess of 75 miles per hour, for example, the vessel hull is supported, in large measure, aerodynamically. The only vessel contact with the water support surface is an extremely small area planing pad at the vessel transom.
For running at speeds in this realm, a vessel is preferably "trimmed" to set the propeller thrust axis in the plane of the vessel planing pad. As a direct consequence, half or less than half of the propeller rotational circle is submerged. The skeg, which is leading the propeller through the water, is therefore essential for lateral stability as well as propeller counter-torque and directional control. Directional control also includes opposition to propeller induced yaw moments. The trailing edge of the skeg is given a small cant from planar alignment with the propeller thrust axis for production of a counter yaw-force.
Structural failure of the skeg at high speed can precipitate disastrous consequences. Consequently, the traditional industry manufacturing practice of integrally casting the skeg and lower gear case shell from weaker grades of casting aluminum that are selected more for a low casting temperature and a smoothly finished surface than for strength and toughness is disturbing to those who operate their equipment in these high speed realms.
From another perspective, at high planing speed the skeg profile area, projected into the propeller thrusting arc, represents a significant proportion of the emersed propeller arc. The degree of such proportion is enlarged by the greater skeg sectional thickness required as a consequence of inherently weak fabrication materials. Hence, the magnitude of power robbing drag imposed by the skeg frontal section area is exponentially amplified due to weak fabrication materials.
Furthermore, this skeg profile projection greatly reduces the propeller drive efficiency over the propeller rotational arc past the skeg projection. In brief, the prior art methods of skeg construction disturbs the water ahead of the propeller arc. At these speeds, the result of this disturbance is a turbulent wake behind the skeg. When the propeller blade engages the turbulently disturbed increment of water behind the skeg, thrust efficiency declines.
In other words, the turbulent slip stream left behind the skeg carries a wake of microeddys and counterflows that were generated and energized by passing around the skeg surface. When the propeller blade engages this wake stream, a certain portion of the fluid in that wake has been thrust into directions of high energy movement contrary to the propeller blade pitch bias. Consequently, the acceleration vectors of the propeller activated fluid mass are directionally dispersed thereby reducing the reaction forces along the propeller thrust axis.
Additionally, this turbulent disturbance of the propeller thrust efficiency occurs at the most inopportune position in the semicircular propeller thrust arc. Vertically beneath the gear case, the propeller rotational arc has just attained maximum efficiency by cutting into undisturbed water with a fully wetted blade. At the water surface, the blade enters the liquid body from a gaseous body (atmosphere) thereby carrying a compressible gas surface coating on the blade into the incompressible fluid mass. As the gas is purged from the blade proximity and surface by water displacement, some slippage occurs to diminish the propeller efficiency over that increment of the already reduced proportion arc. Beyond the surface disturbance arc but before the skeg wake, the propeller blade reaches maximum thrust efficiency. When the propeller blade enters the skeg wake, this maximum thrust is instantly compromised and reduced. After passing the skeg wake, the propeller blade no sooner sheds the skeg induced microturbulence than advance elements of the propeller blade root start to rotationally rise out of the undisturbed water.
With respect to a more subtle function of a high speed, outboard drive unit skeg, the dynamics of a particular submerged propeller arc are that the propeller produces more propulsive thrust on one side of the propeller axis than on the other. This asymmetric thrust necessarily induces a yaw moment. Untrimmed, propeller induced yaw moment must be corrected by a cant in the propulsion axis to the direction of travel. This cant in the propeller thrust axis induces additional drag, power consumption and reduced speed. More efficiently, propeller induced yaw is corrected by a slight steerage curl in the vertical trailing edge of the skeg. The direction of the steerage curl is determined by the propeller rotational direction. The degree of steerage curl for a particular equipment combination is somewhat more ambiguous. Moreover, counter yaw skeg curl adjustment by trial and error is frustrated by the fact that the cast aluminum fabrication materials have low properties of yield and ductility. Excess or repeated bending on the skeg structure results in a fracture. Hence yaw control curl must be cast into a cast aluminum skeg. Finding the optimum degree of yaw control curl for a particular combination of boat, engine and propeller can be a frustrating and expensive quest.
Another source of high speed wake turbulence from an outboard marine drive into the propeller arc surprisingly comes from the engine cooling water inlets. Traditionally, these inlets are one or more small apertures, 2 to 4 holes of about 1/4 in. diameter, for example, in the frontal surface of the drive unit gear case that channel pickup water into an engine cooling water supply pump. Forward velocity of the gear case drives water into the apertures and generates a substantial dynamic pressure head into the engine coolant pump suction port. Cooling water discharge from the pump is channeled into a pipe located internally of the drive shaft housing. Water from the pump discharge pipe is delivered to the engine cooling jackets.
Since these water inlets represent surface discontinuities on the gear case, water flowing past an inlet but not entering the inlet is directionally disrupted. This directional disruption consequently initiates a turbulent wake that follows the gear case surface into the propeller arc.
It is, therefore, an object of the present invention to position the skeg under the gear case at a location that maximizes the arc of maximum blade thrust efficiency.
Another object of the invention is to increase the area of undisturbed water available to the propeller.
Still another object of the invention is to reduce the skeg profile area.
A still further object of the invention is to provide a slimmer yet stronger skeg structure.
Another object of the invention is to provide a stronger skeg assembly with the gear case.
An additional object of the invention is to provide an easily detachable and replaceable skeg in the event of loss or damage.
Also an object of this present invention is a skeg construction that reduces the magnitude of skeg wake turbulence and drag.
Another object of the invention is removal of an engine cooling water inlet aperture to a less turbulence inducing position on the drive unit gear case.
Another object of the invention is to provide a convenient and flexible means for experimentation with the skeg trim parameters and to maximize the boat performance and efficiency.